Elliotte,
This situation doesn't seem at all unique to xml:lang. There
could be other situations where an attribute of an ancestor
of the embedded elements is considered "in-scope" and such
attributes would need to be added to those elements during
inclusion to preserve the semantics those attributes represent.
While the XInclude specification describes behavior related to
the attributes with such properties in the XML specification
that appear in the base Infoset, these are not exhaustive. It
is certainly possible that other specifications could build
upon the base XInclude and specify additional attributes that
would require such special handling.
Removing the requirements for xml:lang processing in XInclude
would appear to send a signal that we are precluding such
usage, something that one tries to avoid in producing a core
specification that is intended for use in as wide a range of
XML processing scenarios as possible.